A Raman amplifier is a type of optical amplifier used in fiber optic transmission systems. Raman amplification is based on the Stimulated Raman Scattering (SRS) phenomenon where a lower frequency signal photon induces a scattering of a higher frequency pump photon in an optical medium in the nonlinear regime. As a result of this, another signal photon is produced, wherein the surplus energy resonantly passes through the vibrational states of the medium. Raman amplifiers are being deployed in long-haul, regional, and metro-core fiber optic transmission systems. Distributed Raman amplifiers improve the optical signal-to-noise ratio and reduce the nonlinear penalty of fiber systems, allowing for longer reach, longer amplifier spans, higher bit rates, higher number of channels, closer channel spacing and operation near the fiber zero dispersion wavelength. Raman scattering of incoming light with phonons in the lattice of the gain medium produces photons coherent with the incoming photons. In a Raman amplifier the optical signal is amplified by Raman amplification. Unlike the erbium-doped fiber amplifiers (EDFA) or the semiconductor optical amplifiers (SOA) the amplification effect of a Raman amplifier is achieved by a nonlinear interaction between a signal and a pump laser within an optical fiber. Two different types of Raman amplifiers are known, i.e. a distributed and a lumped amplifier. The distributed Raman amplifier is a Raman amplifier in which the transmission line optical fiber is utilized as the gain medium by multiplexing a pump wavelength with a signal wavelength whereas a lumped Raman amplifier utilizes a dedicated, self-contained shorter length of optical fiber to provide amplification. In the case of a lumped Raman amplifier a highly nonlinear fiber with a small core such as dispersion compensating fiber is utilized to increase the interaction between the signal and the pump wavelength. The pump light may be coupled into the transmission fiber in the same direction as the signal (co-directional pumping), in the opposite direction (counter-directional pumping) or in both directions (bi-directional pumping).
In a conventional optical transmission system large signal variations may occur when data channels are added or are dropped within the network. This can lead to sudden and large signal power variations at the input of the transmission line fiber connected to the Raman amplifier which in turn causes a change in signal gain of the amplified surviving data channels along the transmission line fiber. Accordingly, there is a need for a fast dynamic signal gain control of co- or counter-pumped distributed Raman amplifiers.